Before I do this I just want to make sure I am not missing something. I used all capacitors that are well over 18v so I am safe to try my Median build at 18v, yes?
Also, what result would you expect in a compressor at 18v vs 9v?
Yes. I do this and love the increased headroom/volume. A rule of thumb I operate by for 18v is to ensure the caps are rated for at least twice that, so there's a lot of 50v and up in my builds regardless.